In certain applications, it is desirable to have a bi-directional motor connected to rotate an output shaft in either direction. At the same time, it is undesirable to allow a load connected to the output shaft to transmit an excessive forward or reverse back drive torque to the motor. Bi-directional back drive stopping clutches have been developed to accomplish this desired function. Such clutches include a pair of brake shoes which do not interfere with bi-directional driving forces from the motor, but are moved to engage a stationary brake drum in response to excessive back drive torques. The brake shoes are generally semicircular having arcuate braking surfaces which when actuated engage the brake drum. The brake shoes may include hardened wear resistant inserts at the braking surfaces. The brake shoes are mounted in an opposing relationship to abut along a common boarder. Input pins or flanges on an input yoke engage the brake shoes to hold the shoes together clear of the brake drum as the input yoke is rotatably driven by a suitable drive mechanism such as a motor. Circular openings are formed between the brake shoes at locations equally spaced on opposite sides of the axis of rotation for the input yoke. A pair of output pins extend between these openings and aligned openings on an output member. The output member is connected to an output shaft. So long as the driving force is transferred to the brake shoes from the input yoke and an excessive back drive torque is not imparted to the output member by the output shaft, the brake shoes rotate and input torque is transferred through the output pins to the output member. In the event that an excessive back drive torque is applied to the output member, the pins are caused to skew, thus spreading the abutting brake shoes apart. A small separation of the brake shoes causes the braking surfaces on the brake shoes to engage the brake drum to prevent back driving of the motor. The clutch is effective both to brake reverse feedback in the driving phase against a strongly opposing load and to brake under an excessive forward or aiding load condition which could otherwise cause run-away.
It has been known in the art that the effectiveness of the clutch is influenced by the design of the output pins and the pin openings between the brake shoes. Prior art output pins and pin openings between the brake shoes are shown, for example, in U.S. Pat. Nos. 3,335,831, 3,414,095 and 3,497,044. In the past, the output pins have had a generally cylindrical body with a quasi-spherical head at the end which fits into an output member opening. The spherical head allows the pin to tilt or skew in the output member opening sufficiently to engage the brake without interference between the skewed pins and the output member. The portion of the pin between the brake shoes has been formed with a uniform cylindrical body terminating at a flat end.
It is known that the output pins connected between the brake and the output member operate as levers as they become skewed to activate the brake. The spherical end of each output pin engages the output member and the body of the skewed pin engages the two brake shoes at spaced apart locations. The point nearest the output member where the pin first contacts one of the brake shoes is the fulcrum of the lever. In the past, the leverage ratio has been changed to modify the braking response of the clutch by enlarging the end of the pin opening between the abutting brake shoes adjacent the output member, as shown in U.S. Pat. No. 3,497,044. The enlarged opening end increases the distance between the point of contact of the pin with the output member and the closest point of contact between the pin and a brake shoe. At the same time, the points of contact between the pin and the two brake shoes are moved closer together. Thus, the fulcrum for the pin is moved away from the output member and towards the end of the pin located between the brake shoes when the end of the shoe openings adjacent the output member are enlarged.
There are several disadvantages to enlarging the end of the brake shoe opening to modify the leverage ratio. The leverage ratio must be considered as only a theoretical value for production clutches. The actual ratio in an assembled clutch is the result of the relative position of all the assembled components. The manufacturing tolerances required for each respective component results in an overall total assembly stack up tolerance. This total tolerance varies from one assembly to another. In addition, the total tolerance can vary within each clutch assembly due to internal clearances of the parts. The result of tolerance stack up and clearances can change the leverage ratio by a magnitude of upwards of a 25 to 30 percent difference compared to the theoretical value. This difference, in turn, leads to a variation in clutch torque capacity by the same corresponding percentage difference in leverage ratio. The resulting clutch torque capacity in an assembled clutch could be adjusted only through replacement of the brake shoes with a pair of shoes having a different dimensioned stepped opening. However, if other tolerances on the replacement shoes were not identical to the original shoes, the replacement shoes might not result in the desired torque capacity. The brake shoes are the most expensive part of the clutch and the manufacturing cost is increased when the end of the opening is enlarged. In order to try different leverage ratios to determine the most effective brake response for a particular application, many differently designed pairs of the expensive brake shoes must be manufactured. Another problem associated with the present design is in the design of the fulcrum on the output pin. Since the shoe fulcrum location is a closely held sharp transition location at a step in the brake shoe openings, high unit compressive loads occur at the fulcrum point. The high compressive loads can cause the fulcrum point to fracture, having an adverse affect on the overall performance of the clutch.